This is the "back door" into the IO monad, allowing
IO computation to be performed at any time. For
this to be safe, the IO computation should be
free of side effects and independent of its environment.

If the I/O computation wrapped in unsafePerformIO
performs side effects, then the relative order in which those side
effects take place (relative to the main I/O trunk, or other calls to
unsafePerformIO) is indeterminate. You have to be careful when
writing and compiling modules that use unsafePerformIO:

Use {-# NOINLINE foo #-} as a pragma on any function foo
that calls unsafePerformIO. If the call is inlined,
the I/O may be performed more than once.

Use the compiler flag -fno-cse to prevent common sub-expression
elimination being performed on the module, which might combine
two side effects that were meant to be separate. A good example
is using multiple global variables (like test in the example below).

Make sure that the either you switch off let-floating, or that the
call to unsafePerformIO cannot float outside a lambda. For example,
if you say:
f x = unsafePerformIO (newIORef [])
you may get only one reference cell shared between all calls to f.
Better would be
f x = unsafePerformIO (newIORef [x])
because now it can't float outside the lambda.

This program will core dump. This problem with polymorphic references
is well known in the ML community, and does not arise with normal
monadic use of references. There is no easy way to make it impossible
once you use unsafePerformIO. Indeed, it is
possible to write coerce :: a -> b with the
help of unsafePerformIO. So be careful!

unsafeInterleaveIO allows IO computation to be deferred lazily.
When passed a value of type IO a, the IO will only be performed
when the value of the a is demanded. This is used to implement lazy
file reading, see System.IO.hGetContents.